Cameron was right to break protocol and criticise Trump

It is highly unusual for British prime ministers or US presidents to comment on the merits of contenders for the highest political offices in the US or UK.

Even breaches of the spirit of this have led to transatlantic bitterness. President Reagan said nothing against Labour leader Neil Kinnock prior to 1987’s general election but belittled him by hosting a brief Oval Office visit.

John Major said nothing against Bill Clinton’s presidential bid in 1992, but annoyed the latter because the Tories had assisted the Republican re-election campaign of George HW Bush.

The tycoon is leading the race for the Republican Party nomination, eclipsing moderate and sensible candidates such as Jeb Bush. Mr Bush, whose campaign is floundering, described Mr Trump as “unhinged” on Monday. This is right not merely because of his latest remarks, that Muslims should be banned from entering America, but because he has made many unpleasant comments, including personal insults against rivals, in his campaign bid.

His idiotic Muslims comment badly damages serious points that mainstream voices, on the political left and right, are making about the need to identify the threat from Islamic extremism and describe it honestly as such. There are issues of high Muslim support for violence in countries such as Britain (a BBC poll found that a quarter of Muslims had sympathy for the motives behind the Charlie Hebdo massacre). But it is plain that the great majority of the world’s 1.5 billion Muslims are moderate and peaceful. Extremist attacks in the US have been few proportionate to its three million Islamic population.

Mr Trump’s comparison with the internment of Japanese in the 1940s is muddled, given that America was then in total war with Japan. The west faces a major problem with terrorism, but the dangers are not currently remotely comparable to those facing the Allies in World War Two.